Approximately 50% of Americans, age 30 and over, have periodontal disease. As periodontal disease progresses, inflammation can ultimately lead to irreversible bone and tooth loss. Mitogen-activated protein kinases (MAPKs) modulate the innate host inflammatory response during periodontal disease. Mitogen- activated protein kinase-activated protein kinase 2 (MK2), a downstream target of p38 MAPK, regulates inflammation in macrophages in aggressive periodontal disease animal models. Both macrophages and osteoclasts are derived from a monocyte lineage that expresses surface chemokine receptors critical for chemotaxis from circulation to peripheral sites of infection. Previous results from our project laboratory show that A. actinomycetemcomitans, an aggressive periodontal disease pathogen, and A. actinomycetemcomitans lipopolysaccharide (LPS) activate MK2. In an A. actinomycetemcomitans LPS-driven rat model, MK2 positively regulates inflammatory infiltration and bone loss. Preliminary data support a role for MK2 as a regulator of CXCR4 cell surface expression on CD11bhi-expressing cells, suggesting modulation of chemokine receptor expression may be an important mechanism of decreasing macrophage infiltrates under MK2 inhibition. MK2 signaling is critical for pathologic osteoclastogenesis, as supported by a decrease in LPS-driven osteoclast formation in Mk2-/- CD11blo osteoclast progenitor cells (OCPs) compared to Mk2+/+ (WT). In vivo, MK2 signaling was also critical for A. actinomycetemcomitans-induced bone loss in a murine calvarial model. These data support our hypothesis that MK2 signaling is required for monocyte chemotaxis and subsequent differentiation into macrophages and osteoclasts during host-pathogen interactions. To address the hypothesis, we will use bone marrow transplant to create MK2 chimeric animals followed by an air pouch model to assess the role of hematopoietic MK2 signaling in monocyte chemotaxis under A. actinomycetemcomitans challenge (Aim #1). The mechanism of MK2 signaling in A. actinomycetemcomitans- induced osteoclastogenesis with regards to NFATc1, the master transcription factor of osteoclastogenesis, will be studied in vitro using OCPs. The murine calvarial bone loss model will be used to confirm that MK2 is critical for A. actinomycetemcomitans driven osteoclastogenesis in vivo (Aim #2). This proposed research will support delineation of the impact of MK2 signaling on monocyte migration to the site of local periodontal disease infection and further differentiation into macrophages and osteoclasts.